Fig. 1. Cellular structure of the organ of Corti and determination of orientation of stereociliary bundles. (A) The apical surface of the organ of Corti at P0. The organ of Corti extends in a spiral along the basal-to-apical axis (arrow) of the cochlear duct. A single row of IHC (black) is located closest to the modiolus (inner edge). Adjacent to the IHC is a row of pillar cells (PC, yellow). On the opposite side of the pillar cells, nearer to the stria vascularis (outer edge) are three or four rows of OHC1-OHC3 (black). In both the inner and outer hair cell regions, each hair cell is separated from neighboring hair cells by non-sensory supporting cells (gray). On the apical surface of each hair cell is a stereociliary bundle. At P0 this bundle is comprised of a single tubulin-based kinocilium (red) and a group of actin-based stereocilia (green dots). Stereocilia bundles are uniformly oriented towards the outer edge, and are arranged into a curved (IHC) or V-shaped (OHC) pattern with the kinocilium located at the vertex of the bundle. (B) The orientation of stereociliary bundles (illustrated as in A) was determined relative to a line (broken white line) extending perpendicular to the row of pillar cells (PC, yellow). If the vertex of the bundle aligned exactly with the perpendicular line, then the bundle was assigned an orientation of 0°. Deviations towards the apex of the cochlea were assigned a positive value and deviations towards the base were assigned a negative value. The OHC on the left has a relatively small deviation of 15° from the perpendicular line, while the OHC on the right has a larger deviation of 70°.

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